1. Field of the Invention
The present invention relates to imaging various media or objects using scattered, reflected, and/or transmitted radiation in the terahertz (THz) region of the electromagnetic spectrum and, more particularly, to optical detection of THz radiation.
2. Description of the Related Art
The term “terahertz radiation” refers herein to electromagnetic radiation having wavelengths in a range between about 10 μm and about 10 mm. Terahertz (THz) radiation can penetrate well most nonmetallic objects, such as paper, cardboard, plastics, and moderate thickness of many dielectrics, while being blocked or absorbed by metals and polar materials. As a result, the THz spectral range is becoming increasingly important for such applications as remote sensing of gases, quality control of plastic and composite materials, package inspection, moisture analysis, etc. A relatively recent development is the use of THz radiation for imaging, often referred to as T-ray imaging. Description of representative prior-art T-ray imaging systems can be found, e.g., in commonly owned U.S. Pat. Nos. 5,623,145, 5,710,430, 5,894,125, and 6,078,047, the teachings of all of which are incorporated herein by reference.
One problem with T-ray imaging is that the wavelength (λ) of THz radiation is relatively large compared to, e.g., that of visible light. As a result, the spatial resolution of T-ray images is generally relatively poor, because the spatial resolution is typically related to the wavelength of the interrogating radiation. To address this problem, near-field T-ray imaging techniques have been proposed. In near-field T-ray imaging, the spatial resolution is generally limited not by the value of λ, but by the effective aperture of the imaging apparatus. When an apparatus having a relatively small effective aperture is configured to scan an object illuminated by THz radiation, a T-ray image of the object having a relatively high spatial resolution, e.g., better than the applicable T-ray diffraction limit (i.e., ˜λ/2), can be created.
One convenient method of detecting THz radiation is based on an electro-optic (EO) effect. More specifically, an EO effect causes the refractive index of an electro-optically responsive material to depend on the intensity of an electric field, e.g., that of THz radiation. As a result, light traveling through the electro-optically responsive material acquires a phase retardation related to the light propagation distance and the intensity of the THz field. The latter can therefore be deduced by pumping the electro-optically responsive material with pump light and measuring the pump-light phase retardation.
The use of EO detection in near-field T-ray imaging has however encountered difficulties because, for near-field imaging, a relatively small EO interaction region is used to substantially avoid the near-field to far-field conversion of the THz radiation inside the EO detector. Unfortunately, the relatively small EO interaction region generally causes the phase retardation acquired by the pump light in that region to also be relatively small, which hampers an accurate intensity determination of the THz field.